Chalcogenide devices and materials having reduced germanium or telluruim content

ABSTRACT

A chalcogenide material and memory device exhibiting fast operation over an extended range of reset state resistances. Electrical devices containing the chalcogenide materials permit rapid transformations from the reset state to the set state for reset and set states having a high resistance ratio. The devices provide for high resistance contrast of memory states while preserving fast operational speeds. The chalcogenide materials include Ge, Sb and Te where the Ge and/or Te content is lean relative to Ge 2 Sb 2 Te 5 . In one embodiment, the concentration of Ge is between 11% and 22%, the concentration of Sb is between 22% and 65%, and the concentration of Te is between 28% and 55%. In a preferred embodiment, the concentration of Ge is between 15% and 18%, the concentration of Sb is between 32% and 35%, and the concentration of Te is between 48% and 51%.

RELATED APPLICATION INFORMATION

This application is a continuation in part of U.S. patent applicationSer. No. 11/200,466, entitled “Chalcogenide Devices IncorporatingChalcogenide Materials having Reduced Germanium or Tellurium Content”,filed on Aug. 9, 2005; the disclosure of which is hereby incorporated byreference herein.

FIELD OF INVENTION

This invention pertains to chalcogenide materials having utility inelectrical memory or switching devices. More particularly, thisinvention is concerned with off-tieline chalcogenide alloys in theGe—Sb—Te family that have a low Ge concentration and/or low Teconcentration relative to the widely used Ge₂Sb₂Te₅ alloy. Mostspecifically, this invention relates to electrical chalcogenidematerials that exhibit high set speeds from initial states having highresistances.

BACKGROUND OF THE INVENTION

Chalcogenide materials are an emerging class of commercial electronicmaterials that exhibit switching, memory, logic, and processingfunctionality. The basic principles of chalcogenide materials weredeveloped by S. R. Ovshinsky in the 1960's and much effort by him andothers around the world in the past few decades have led to advancementsin the underlying science that governs the structure and properties ofchalcogenide materials and an expansion of the range of practicalapplication to which chalcogenide materials can be put.

Early work in chalcogenide devices demonstrated an electrical switchingbehavior in which switching from a resistive state to a conductive statewas induced upon application of a voltage at or above a thresholdvoltage. Although the threshold voltage is formally a property of thedevice, the response of the active chalcogenide material to the voltageis the critical factor underlying the magnitude of the thresholdvoltage. The voltage-induced resistive-to-conductive transformation isthe basis of the Ovonic Threshold Switch (OTS) and remains an importantpractical feature of chalcogenide materials. The OTS provides highlyreproducible switching at ultrafast switching speeds for over 10¹³cycles. Basic principles and operational features of the OTS arepresented, for example, in U.S. Pat. Nos. 3,271,591; 5,543,737;5,694,146; and 5,757,446; the disclosures of which are herebyincorporated by reference, as well as in several journal articlesincluding “Reversible Electrical Switching Phenomena in DisorderedStructures”, Physical Review Letters, vol. 21, p. 1450-1453 (1969) by S.R. Ovshinsky; “Amorphous Semiconductors for Switching, Memory, andImaging Applications”, IEEE Transactions on Electron Devices, vol.ED-20, p. 91-105 (1973) by S. R. Ovshinsky and H. Fritzsche; thedisclosures of which are hereby incorporated by reference.

Other important applications of chalcogenide materials includeelectrical and optical memory devices. One type of chalcogenide memorydevice utilizes the wide range of resistance values available for thematerial as the basis of memory operation. Each resistance valuecorresponds to a distinct structural state of the chalcogenide materialand one or more of the states can be selected and used to defineoperational memory states. Chalcogenide materials exhibit a crystallinestate or phase as well as an amorphous state or phase. Differentstructural states of a chalcogenide material differ with respect to therelative proportions of crystalline and amorphous phase in a givenvolume or region of chalcogenide material. The range of resistancevalues is bounded by a set state and a reset state of the chalcogenidematerial. The set state is a low resistance structural state whoseelectrical properties are primarily controlled by the crystallineportion of the chalcogenide material and the reset state is a highresistance structural state whose electrical properties are primarilycontrolled by the amorphous portion of the chalcogenide material.

Each memory state of a chalcogenide memory material corresponds to adistinct resistance value and each memory resistance value signifiesunique informational content. Operationally, the chalcogenide materialcan be programmed into a particular memory state by providing anelectric current pulse of appropriate amplitude and duration totransform the chalcogenide material into the structural state having thedesired resistance. By controlling the amount of energy provided to achalcogenide material, it is possible to control the relativeproportions of crystalline and amorphous phase regions within a volumeof the material and to thereby control the structural (and memory) stateof the chalcogenide material.

Each memory state can be programmed by providing the current pulsecharacteristic of the state and each state can be identified or read ina non-destructive fashion by measuring the resistance. Programming amongthe different states is fully reversible and the memory devices can bewritten and read over a virtually unlimited number of cycles to providerobust and reliable operation. The variable resistance memoryfunctionality of chalcogenide materials is currently being exploited inthe OUM (Ovonic Universal (or Unified) Memory) devices that arebeginning to appear on the market. Basic principles and operation of OUMtype devices are presented, for example, in U.S. Pat. Nos. 6,859,390;6,774,387; 6,687,153; and 6,314,014; the disclosures of which areincorporated by reference herein as well as in several journal articlesincluding “Low Field Amorphous State Resistance and Threshold VoltageDrift in Chalcogenide Materials”, published in IEEE Transactions onElectron Devices, vol. 51, p. 714-719 (2004) by Pirovana et al.; and“Morphing Memory” published in IEEE Spectrum, vol. 167, p. 363-364(2005) by Weiss.

The general behavior (including switching, memory, and accumulation) andchemical compositions of chalcogenide materials have been described, forexample, in the following U.S. Pat. Nos. 6,671,710; 6,714,954;6.087,674; 5,166,758; 5,296,716; 5,536,947; 5,596,522; 5,825,046;5,687,112; 5,912,839; and 3,530,441, the disclosures of which are herebyincorporated by reference. These references also describe proposedmechanisms that govern the behavior of the chalcogenide materials,including the structural transformations from a crystalline state to anamorphous state (and vice versa) via a series of partially crystallinestates that underlie much of the operational characteristics ofelectrical and optical chalcogenide materials.

Current commercial development of the chalcogenide materials and devicesis also oriented toward the fabrication of arrays of devices.Chalcogenide materials offer the promise of high density memory, logicand neural arrays that can operate using either a traditional binarydata storage protocol or a non-binary, multilevel protocol. Chalcogenidearrays further offer the prospect of integrating, on a single chip, bothmemory and processing functionality.

In order to further expand the commercial prospects of chalcogenidephase change memories, it is necessary to devise further improvements inthe chemical and physical properties of chalcogenide materials as wellas in manufacturing processes. A current issue in terms of theproperties of chalcogenide materials is the need to improve the thermalstability of the materials. Data in a chalcogenide material are retainedas a structural state of the material, so any tendency of the structuralstate to transform with temperature represents a potential undesirablemechanism of erasing or losing data. Many chalcogenide memory materialsretain their structural states for long periods of time at roomtemperature, but become susceptible to variations in the structuralstate upon increasing temperature. In practical terms, this limits thetemperature environment in which chalcogenide memory devices can beutilized as well as the temperatures that can be employed in processingor manufacturing. It is desirable to develop new chalcogenidecompositions having structural states that are stable over anever-increasing range of temperatures.

In most currently-envisioned memory applications, chalcogenide materialsare operated in a binary mode where the memory states correspond to, orapproximately correspond to, the set state and the reset state sincethese states provide the greatest contrast in resistance and thusfacilitate discrimination of the state of the material during read out.In most of the fabrication processes contemplated for commercialproduction of chalcogenide memory devices, the chalcogenide material isdeposited on a substrate; electrical contact layer or other layer. Afterdeposition the chalcogenide material is in an amorphous or otherwisedisordered state and is converted to a crystalline state duringsubsequent processing. In completed, fully fabricated devices, it issometimes necessary to electrically exercise or “form” the chalcogenidematerial to ready the device for consistent operation as the activematerial of a memory element. The formation process includes the step oftransforming the as-processed chalcogenide device to the optimum statefor product use. In devices that employ the widely used Ge₂Sb₂Te₅ alloy,the formation process requires multiple cycles of setting and resettingto achieve a set state resistance that stabilizes to a desirable andreproducible value.

In order to increase the efficiency of manufacturing, it is desirable todevelop chalcogenide materials and device structures that can beelectrically conditioned for practical operation in the minimum time. InU.S. patent application Ser. No. 11/200,466 (the '466 application), theinstant inventors identified a series of new chalcogenide compositionsthat required little or no formation. The alloys include Ge and a columnV element, where the column V element is preferably Sb. In someembodiments, the alloys further included Te. Relative to the widely-usedGe₂Sb₂Te₅ composition, the alloys were lean in Ge and/or Te. The alloysof the '466 application may be referred to as “off-tieline” alloysbecause the compositions of the alloys are located away from the tielineconnecting Sb₂Te₃ and GeTe on a ternary Ge—Sb—Te phase diagram.

In addition to less stringent post-processing formation requirements, itis further desirable to develop chalcogenide alloys that exhibit fastcrystallization speeds over a series of memory states that extend over awide dynamic range of resistance.

SUMMARY OF THE INVENTION

In one embodiment, the instant invention provides chalcogenide alloycompositions that exhibit favorable formation characteristics along withshort crystallization times. In another embodiment, the instantinvention provides chalcogenide alloy compositions that exhibitcrystallization times that vary only slightly over different structuralstates whose resistances extend over one or more orders of magnitude.When used in electrical chalcogenide device applications, the instantalloys provide for fast set speeds and/or favorable formationcharacteristics for a plurality of states that extend over a wide rangeof resistance. The instant alloys also provide favorable thresholdvoltages, reset currents, and reset resistances.

The instant alloys generally comprise Ge, Sb, and/or Te where the atomicconcentration of Ge is generally in the range from 11%-22%, the atomicconcentration of Sb is generally in the range from 22%-65%, and theatomic concentration of Te is generally in the range from 28%-55%. Inone embodiment, the instant alloys include an atomic concentration of Gein the range from 13%-20%, an atomic concentration of Sb in the rangefrom 28%-43%, and an atomic concentration of Te in the range from43%-55%. In another embodiment, the instant alloys include an atomicconcentration of Ge in the range from 15%-18%, an atomic concentrationof Sb in the range from 32%-35%, and an atomic concentration of Te inthe range from 48%-51%.

The instant invention includes electrical devices containing the instantchalcogenide materials where the devices include a layer of chalcogenidematerial in electrical communication with two electrical terminals orcontacts. The instant invention further includes arrays of such devices.In one embodiment, a device including one of the instant alloys requiresa set pulse time of less than 100 ns when the reset resistance is ≦200kΩ. In another embodiment, a device including one of the instant alloysrequires a set pulse time of less than 40 ns when the reset resistanceis ≦100 kΩ. In a preferred embodiment, a device including one of theinstant alloys requires a set pulse time of less than 20 ns when thereset resistance is ≦40 kΩ. In a preferred embodiment, a deviceincluding one of the instant alloys requires a set pulse time of lessthan 30 ns when the reset resistance is ≦60 kΩ.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic depiction of the resistance of a chalcogenide materialas a function of energy or current.

FIG. 2. The variation of set pulse width as a function of resetresistance of several two-terminal electrical devices includingdifferent chalcogenide alloys according to the instant invention.

FIG. 3. Set pulse width of a two-terminal electrical device from a resetstate having a resistance of above 100 kΩ to a set state having aresistance below 5 kΩ as a function of the atomic concentrations of Ge,Sb and Te present in the active chalcogenide material of the device.

FIG. 4. Reset current of a two-terminal electrical device to a saturatedreset state as a function of the atomic concentrations of Ge, Sb and Tepresent in the active chalcogenide material of the device.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The instant invention provides chalcogenide materials and electricaldevices containing the instant chalcogenide materials that exhibitfavorable operating characteristics for practical memory and switchingapplications. Devices that include the instant alloys exhibit short settimes from reset states having high resistances. The devices providememory states that span a wide range of resistances where each memorystate exhibits a short set time. The devices thus permit rapidtransformations between memory states that differ widely in resistance.In particular, devices including the instant chalcogenide alloys enabletransformations between a high resistance memory state and a lowresistance memory state in which both the speed of the transformationand the ratio of the resistances of the two states is high. The instantdevices further provide favorable threshold voltages and reset currents.

The devices also possess favorable forming characteristics similar tothose described in the '466 application for similar alloy compositions.In some embodiments, devices that include the instant chalcogenidematerials require no forming after fabrication to condition the devicefor practical application. In these embodiments, the set resistance isstable upon cycling between the set and reset states immediatelyfollowing deposition of the device and the stabilized set resistancedeviates only slightly from the virgin resistance of the device. As aresult, the need for post-processing electrical exercise of the devicesprior to practical use is greatly reduced. Additional information on theforming process can be found in the '466 application.

Since the underlying basis of the improved set speed characteristics ofthe instant alloys is related to the structural characteristics ofchalcogenide materials, it is helpful to review the basic principles ofoperation of chalcogenide materials. An important feature of thechalcogenide materials during the operation of chalcogenide memorydevices and device arrays is their ability to undergo a phasetransformation between or among two or more structural states. (Theimportance of phase transformations in memory applications has promptedsome people to refer to chalcogenide materials as phase change materialsand they may be referred to herein as such.) The chalcogenide materialshave structural states that include a crystalline state, one or morepartially-crystalline states and an amorphous state. The crystallinestate may be a single crystalline state or a polycrystalline state. Asused herein, a partially-crystalline state refers to a structural stateof a volume of chalcogenide material that includes an amorphous portionand a crystalline portion. Generally, a plurality ofpartially-crystalline states exists for the phase-change material thatmay be distinguished on the basis of the relative proportion of theamorphous and crystalline portions. Fractional crystallinity is one wayto characterize the structural states of a chalcogenide phase-changematerial. The fractional crystallinity of the crystalline state is 100%,the fractional crystallinity of the amorphous state is 0%, and thepartially-crystalline states have fractional crystallinities that varycontinuously between 0% (the amorphous limit) and 100% (the crystallinelimit). Phase-change chalcogenide materials are thus able to transformamong a plurality of structural states that vary inclusively betweenfractional crystallinities of 0% and 100%.

Transformations among the structural states of a chalcogenide materialare induced by providing energy to the chalcogenide material. Energy invarious forms can influence the fractional crystallinity of achalcogenide material and induce structural transformations. Suitableforms of energy include electrical energy, thermal energy, opticalenergy or other forms of energy (e.g. particle-beam energy) that induceelectrical, thermal or optical effects in a chalcogenide material.Combinations of different forms of energy may also induce structuraltransformations. Continuous and reversible variability of the fractionalcrystallinity is achievable by controlling the energy environment of achalcogenide material. A crystalline state can be transformed to apartially-crystalline or an amorphous state, a partially-crystallinestate can be transformed to a different partially-crystalline state aswell as to either a crystalline or amorphous state, and an amorphousstate can be transformed to a partially-crystalline or crystalline statethrough proper control of the energy environment of a chalcogenidematerial. Some considerations associated with the use of thermal,electrical and optical energy to induce structural transformations arepresented in the following discussion.

The use of thermal energy to induce structural transformations exploitsthe thermodynamics and kinetics associated with the crystalline toamorphous or amorphous to crystalline phase transitions. An amorphousstate may be formed, for example, from any prior state (including apartially-crystalline, crystalline or amorphous state) by heating achalcogenide material above its melting temperature and cooling at arate sufficient to inhibit the formation of crystalline phases. Acrystalline state may be formed from any prior state (including apartially-crystalline, crystalline or amorphous state), by, for example,heating a chalcogenide material above the crystallization temperaturefor a sufficient period of time to effect nucleation and/or growth ofcrystalline domains. The crystallization temperature is below themelting temperature and corresponds to a temperature at whichcrystallization may occur. The driving force for crystallization istypically thermodynamic in that the free energy of a crystalline orpartially-crystalline state is lower than the free energy of anamorphous state so that the overall energy of a chalcogenide materialdecreases as the fractional crystallinity increases. Formation(nucleation and growth) of a crystalline state or crystalline domainswithin a partially-crystalline state is kinetically enabled, so thatheating below the melting point promotes crystallization by providingenergy that facilitates the rearrangements of atoms needed to form acrystalline phase or domain. The fractional crystallinity of apartially-crystalline state can be controlled by controlling thetemperature or time of heating of the previously amorphous chalcogenidematerial or by controlling the temperature or rate of cooling of thepreviously amorphous chalcogenide material.

The use of electrical energy to induce structural transformationstypically relies on the application of electrical (current or voltage)pulses to a chalcogenide material. By controlling the magnitude and/orduration of electrical pulses applied to a chalcogenide material, it ispossible to continuously vary the fractional crystallinity. Theinfluence of electrical energy on the structure of a chalcogenidematerial is frequently depicted in terms of the variation of the lowfield electrical resistance of a chalcogenide material with the amountof electrical energy provided or the magnitude of the current or voltagepulse applied to a chalcogenide material. A representative depiction ofthe low field electrical resistance (R) of a chalcogenide material as afunction of electrical energy or current pulse magnitude(Energy/Current) is presented in FIG. 1 herein. FIG. 1 shows thevariation of the low field electrical resistance of a chalcogenidematerial resulting from electrical energy or current pulses of variousmagnitude and may generally be referred to as a resistance plot.

The resistance plot includes two characteristic response regimes of achalcogenide material to electrical energy. The regimes areapproximately demarcated with the vertical dashed line 10 shown inFIG. 1. The regime to the left of the line 10 may be referred to as theaccumulating regime of the chalcogenide material. The accumulationregime is distinguished by a nearly constant or gradually varyingelectrical resistance with increasing electrical energy that culminatesin an abrupt decrease in resistance at and beyond a threshold energy.The accumulation regime thus extends, in the direction of increasingenergy, from the leftmost point 20 of the resistance plot, through aplateau region (generally depicted by 30) corresponding to the range ofpoints over which the resulting resistance variation is small or gradualup to the set point or state 40 that follows an abrupt decrease inelectrical resistance. The plateau 30 may be horizontal or sloping. Theleft side of the resistance plot is referred to as the accumulatingregime because the structural state of the chalcogenide materialcontinuously evolves as energy is applied, with the fractionalcrystallinity of the structural state correlating with the totalaccumulation of applied energy. The leftmost point 20 corresponds to thestructural state in the accumulating regime having the lowest fractionalcrystallinity. This state may be fully amorphous or may contain someresidual crystalline content. As energy is added, the fractionalcrystallinity increases, and the chalcogenide material transforms in thedirection of increasing applied energy among a plurality ofpartially-crystalline states along the plateau 30. Selected accumulationstates (structural states in the accumulation region) are marked withsquares in FIG. 1. Upon accumulation of a threshold amount of appliedenergy, the fractional crystallinity of the chalcogenide materialincreases sufficiently to effect a setting transformation that ischaracterized by a dramatic decrease in electrical resistance andstabilization of the set state 40. Structural transformations in theaccumulating regime are unidirectional in the sense that they progressin the direction of increasing applied energy within the plateau region30 and are reversible only by first amorphizing or resettingchalcogenide material. The behavior illustrated in FIG. 1 isreproducible over many cycles of setting and resetting a devicecontaining a chalcogenide material by applying the requisite energy orcurrent. Once the reset state is obtained, lower amplitude currentpulses can again be applied and the accumulation response of thechalcogenide material can be retraced. It is thus possible to cyclebetween the set and reset states over multiple cycles, a necessaryfeature for high memory cycle life.

While not wishing to be bound by theory, the instant inventors believethat the addition of energy to a chalcogenide material in theaccumulating regime leads to an increase in fractional crystallinitythrough the nucleation of new crystalline domains or growth of existingcrystalline domains or a combination thereof. It is believed that theelectrical resistance varies only gradually along the plateau 30 despitethe increase in fractional crystallinity because the crystalline domainsform or grow in relative isolation of each other so as to prevent theformation of a contiguous crystalline network that spans thechalcogenide material between the two device electrodes. This type ofcrystallization may be referred to as, sub-percolation crystallization.The setting transformation coincides with a percolation threshold inwhich a contiguous, interconnected crystalline network forms within thechalcogenide material between the two device electrodes. Such a networkmay form, for example, when crystalline domains increase sufficiently insize to impinge upon neighboring domains. Since the crystalline phase ofchalcogenide materials is less resistive than the amorphous phase, thepercolation threshold corresponds to the formation of a contiguous lowresistance conductive pathway through the chalcogenide material. As aresult, the percolation threshold is marked by a dramatic decrease inthe resistance of the chalcogenide material. The leftmost point of theaccumulation regime may be an amorphous state or a partially-crystallinestate lacking a contiguous crystalline network. Sub-percolationcrystallization commences with an initial amorphous orpartially-crystalline state and progresses through a plurality ofpartially-crystalline states having increasingly higher fractionalcrystallinities until the percolation threshold is reached and thesetting transformation occurs.

The regime to the right of the line 10 of FIG. 1 may be referred to asthe grayscale regime or grayscale region. The grayscale regime extendsfrom the set state 40 through a plurality of intermediate states(generally depicted by 50) to a reset point or state 60. The variouspoints in the grayscale regime may be referred to as grayscale states ofthe chalcogenide material. Selected grayscale states are marked withcircles in FIG. 1. Structural transformations in the grayscale regimemay be induced by applying an electric current or energy pulse to achalcogenide material, as indicated in FIG. 1. In the grayscale regime,the resistance of the chalcogenide material varies with the magnitude ofthe applied electric pulse. The resistance of a particular state in thegrayscale regime is characteristic of the structural state of thechalcogenide material, and the structural state of a chalcogenidematerial is dictated by the magnitude of the current pulse applied inthe grayscale region. The fractional crystallinity of the chalcogenidematerial decreases as the magnitude of the current pulse increases. Thefractional crystallinity is highest for grayscale states at or near theset point 40 and progressively decreases as the reset state 60 isapproached. The chalcogenide material transforms from a structural statepossessing a contiguous crystalline network at the set state 40 to astructural state that is amorphous or substantially amorphous orpartially-crystalline without a contiguous crystalline network at thereset state 60. The application of current pulses having increasingmagnitude has the effect of converting portions of the crystallinenetwork into an amorphous phase and ultimately leads to a disruption orinterruption of contiguous high-conductivity crystalline pathways in thechalcogenide material. As a result, the resistance of the chalcogenidematerial increases as the magnitude of an applied current pulseincreases in the grayscale region.

In contrast to the accumulating region, structural transformations thatoccur in the grayscale region are reversible and bidirectional. For thisreason, the grayscale region may also be referred to as the directoverwrite region of the resistance plot. As indicated hereinabove, eachstate in the grayscale region may be identified by its resistance and acurrent pulse magnitude, where application of that current pulsemagnitude induces changes in fractional crystallinity that produce theparticular resistance value of the state. Application of a subsequentcurrent pulse may increase or decrease the fractional crystallinityrelative to the fractional crystallinity of the initial state of thechalcogenide material. If the subsequent current pulse has a highermagnitude than the pulse used to establish the initial state, thefractional crystallinity of the chalcogenide material decreases and thestructural state is transformed from the initial state in the directionof the higher resistance reset state along the grayscale resistancecurve. Similarly, if the subsequent current pulse has a lower magnitudethan the pulse used to establish the initial state, the fractionalcrystallinity of the chalcogenide material increases and the structuralstate is transformed from the initial state in the direction of thelower resistance set state along the grayscale resistance curve.

In OUM (Ovonic Unified (or Universal) Memory) applications, thegrayscale states of the chalcogenide material are used to define memorystates of a memory device. Most commonly, the memory devices are binarymemory devices that utilize-two of the grayscale states as memorystates, where a distinct information value (e.g. “0” or “1”) isassociated with each state. Each memory state thus corresponds to adistinct structural state of the chalcogenide material and readout oridentification of the state can be accomplished by measuring theresistance of the material (or device) since each structural state ischaracterized by a distinct resistance value as exemplified, forexample, by the grayscale states in FIG. 1. The operation oftransforming a chalcogenide material to the structural state associatedwith a particular memory state may be referred to herein as programmingthe chalcogenide material, writing to the chalcogenide. material orstoring information in the chalcogenide material.

To facilitate readout and to minimize readout error, it is desirable toselect the memory states of a binary memory device so that the contrastin resistance of the two states is large. Typically the set state (or astate near the set state) and the reset state (or a state near the resetstate) are selected as memory states in a binary memory application. Theresistance contrast depends on details such as the chemical compositionof the chalcogenide, the thickness of the chalcogenide material in thedevice and the geometry of the device. For a layer of phase-changematerial having the composition Ge₂₂Sb₂₂Te₅₆, a thickness of ˜600 Å, andpore diameter of below ˜0.1 μm in a typical two-terminal devicestructure, for example, the resistance of the reset state is ˜100-1000kΩ and the resistance of the set state is under ˜10 kΩ. Phase-changematerials in general show resistances in the range of ˜100 kΩ to ˜1000kΩ in the reset state and resistance of ˜0.5 kΩ to ˜50 kΩ in the setstate. In the preferred phase-change materials, the resistance of thereset state is at least a factor-of two, and more typically an order ofmagnitude or more, greater than the resistance of the set state. Inaddition to binary (single bit per device) memory applications,chalcogenide materials may be utilized as non-binary or multiple bit perdevice memory devices by selecting three or more states from among thegrayscale states and associating an information value with each state,where each memory state corresponds to a distinct structural state ofthe chalcogenide and is characterized by a distinct resistance value.

One embodiment of the instant invention provides chalcogenide materialsthat enable devices having improved operational speeds, where devicespeed refers to the time required to induce transformations betweenstructural states. As described hereinabove, the storage of informationin a chalcogenide material entails a process in which energy is appliedto the chalcogenide material to induce a structural transformation tothe memory state that represents the item of information that the userwishes to store. The speed of the device is governed by the rate atwhich the structural transformations occur and this rate ultimatelydepends on the kinetics of the transformations between the crystallineand amorphous (or vice versa) states of the chalcogenide material. Froma phenomenological viewpoint, structural transformations that induce anincrease in fractional crystallinity are expected to be slower thanstructural transformations that induce a decrease in fractionalcrystallinity. This expectation follows because the formation of acrystalline phase from an amorphous phase requires the establishment ofan ordered phase from a disordered phase and the achievement of anordered phase necessarily entails an atomic rearrangement that requiressignificant repositioning of atoms over one or more atomic distances.The time scales required for the necessary atomic motions andreorientation of the atomic bonds in a periodic ordered array isrequired and the process of crystallization is necessarily anequilibrium process.

A structural transformation that leads to a decrease in fractionalcrystallinity, in contrast, is an inherently non-equilibrium processthat readily occurs on time scales shorter than the equilibrium timescale associated with crystallization of the same material. A decreasein fractional crystallinity involves the transformation of an orderedcrystalline region into a disordered amorphous region. The crystallineregion is first melted and then quenched to form an amorphous phase. Themelting process is not limited by timescales associated with atomicmotion of the material and the quenching process generally occurs on ashorter time scale than the crystallization process. The rate ofaddition of energy to cause melting and the rate of removal of energy toquench the melted state determine the timescale of the transformation.Both rates can be controlled by external experimental conditions and canoccur on extremely short time scales. In typical chalcogenide materials,the equilibrium timescale associated with the crystallization process istypically in the 10-1000 nanosecond regime, while the amorphization(melting-quenching) process occurs on sub-nanosecond to 10 nanosecondtimescales and can be made to occur on picosecond and even femtosecondtimescales by controlling the experimental conditions.

From the above discussion, it follows that the speed of operation of achalcogenide memory device is determined in part by the rate oftransformations from states having a low fractional crystallinity tostates having a high fractional crystallinity. In the typical binarydevice, the two memory states are the reset state (a state having a lowfractional crystallinity) and the set state (a state having a highfractional crystallinity) and the speed of operation is expected to besignificantly influenced by the timescale of the transformation from thereset state to the set state.

In addition to the rate of the transformation from an amorphous phase toa crystalline phase, the time required to set a chalcogenide devicedepends on the crystalline volume fraction of the initial state of thedevice. As described hereinabove in connection with FIG. 1, memorystates of a chalcogenide device can be selected from among the grayscalestates 50 that extend from the set state 40 to the reset state 60. Theset state 40 can be formed from either the reset state 60 or any of thegrayscale states 50 by applying and accumulating increments of energy asdescribed hereinabove to achieve a percolation condition. The magnitudeof the energy increments required to achieve the percolation needed toset one of the grayscale states is less than the energy needed to inducea transformation from the reset state in the region of 60 of FIG. 1.

Grayscale states that are in close proximity to the set state have aresistance and fractional crystallinity that are similar to that of theset state. These states have a relative low resistance and require arelatively low net accumulation of energy to undergo the settingtransformation. The similarity in the fractional crystallinity of lowresistance grayscale states to the set state implies a structuralconfiguration or condition for low resistance grayscale scale statesthat is not far removed from the percolated configuration of the setstate. Since only a minor transformation in the structural configurationis needed to achieve the set state, the time required to induce thesetting transformation is shorter and more rapid transitions between lowresistance grayscale states and the set state are possible.

As the resistance of a grayscale state increases, however, thestructural configuration becomes less crystalline and deviates moresignificantly from the percolated configuration of the set state. Agreater accumulation of energy and more substantial transformation ofthe structural configuration is needed to achieve the set state. Thetimescale of the setting transformation increases accordingly. Thisleads to slower operational speeds for memory devices that utilize highresistance grayscale states as memory states.

The foregoing discussion indicates that the intrinsic rate ofcrystallization of the working chalcogenide material from an amorphousor low fractional crystallinity state and the initial state ofcrystallinity of the chalcogenide material are two important factorsthat govern the rate of transformation between memory states and hence,the speed of operation of a memory device. While it is true that fastoperational speeds can be achieved by selecting memory states havingstructural configurations not far removed from the percolatedconfiguration of the set state, this approach is inadequate in manyinstances. The main drawback arises because in such an approach, theresistance contrast between different memory states is low and-itbecomes more difficult to discriminate among the different memory statesduring read out due to the similarity in the resistances of thedifferent states.

In this invention, a more effective approach to achieving fasteroperational speeds is realized through chalcogenide compositions thatexhibit improved intrinsic rates of crystallizations. The instantchalcogenide alloys enable chalcogenide memory devices that exhibit fasttransformations to the set state from grayscale states that extend overa wider range of resistances than is possible with the prior art alloys.The fast intrinsic crystallization rate permits rapid transformations tothe set state from high resistance grayscale states because with fastcrystallization, the initial deviation of the structural configurationof a grayscale state from the percolated configuration of the set statebecomes less important in establishing the speed of transformation. Ahigh crystallization rate can compensate for the increased deviation instructural configurations between a high resistance memory state and theset state. The instant chalcogenide alloys thus enable memory devicesthat provide both high operational speeds and high resistance contrastbetween memory states.

While not wishing to be bound by theory, the instant inventors recognizethat the process of crystallization in a chalcogenide material can occurthrough one or more of the following mechanisms: the nucleation ofcrystalline domains from amorphous domains, growth of such nucleatedphases, and the growth of pre-existing crystalline regions. Anenhancement in the speed of one or more of these mechanisms is expectedto increase the crystallization rate. An enhanced nucleation rate wouldprovide an increase in the concentration of crystalline nuclei and sincecrystalline nuclei seed the crystallization process, fastercrystallization rates would result. Growth is a process in which thesize of existing crystalline regions increases through an interfacialconversion of amorphous material to crystalline material at theboundaries of crystalline domains. An enhanced growth rate would promotethe expansion of crystalline domains and facilitate the transformationto a percolated structural configuration.

The formation of a crystalline phase from an amorphous phase isgenerally thermodynamically favored, but kinetically inhibited. Attemperatures below the melting point, the free energy of the crystallinephase is lower than the free energy of the amorphous phase and as aresult, there is a thermodynamic driving force for crystallization. Asindicated above, however, in order to crystallize, it is necessary forthe material to undergo the atomic rearrangements necessary to realizean ordered crystalline state. An energy barrier must be surmounted toinduce the necessary rearrangements and this energy barrier acts toinhibit crystallization. The nucleation and growth processes are bothaccompanied by an energy barrier. The kinetic probability of thecrystallization process decreases as the magnitude of the energy barrierincreases. A possible explanation of the enhanced crystallization ratesobserved in the instant alloys is a reduction in the energy barrierassociated with either or both of the nucleation and growth processes. Areduced energy barrier would occur in chalcogenide compositions thatexhibit facile atomic rearrangements at temperatures between thecrystallization and melting temperatures. Facile rearrangements would beexpected in compositions that have reduced structural rigidity,especially in the amorphous phase.

A possible explanation of the improved crystallization rates may befound in the relative atomic concentrations of the different elements inthe instant compositions. The instant materials generally include Ge,Sb, and Te. These elements are tetravalent, trivalent, and divalent;respectively. In many amorphous chalcogenide phases; Te promotes theformation of extended chain structures and Ge and Sb function asmodifying elements that act to promote crosslinking between the chains.Ge is a highly crosslinking element, while Sb is only a moderatelycrosslinking element. Crosslinking acts to increase the rigidity of theamorphous phase structure, so a reduction in Ge and/or Sb concentrationmay have a tendency to render an amorphous phase less rigid. As the Geand/or Sb concentration is reduced, however, the Te concentrationincreases and this has the effect of promoting chain length. Long chainlengths are disadvantageous from a crystallization perspective becauselong chains are difficult to rearrange to create an ordered state thatis conducive to crystallization. The instant chalcogenide materials aregenerally lean in the atomic concentration of Ge and Te and rich in theatomic concentration of Sb relative to prior art materials. Thereduction in Ge suggests a reduced tendency to form crosslinks in theamorphous phase and may act to promote crystallization through areduction in structural rigidity. A reduction in Te may act to reducethe number and/or length of chain like structures in the amorphous phaseand this may promote crystallization by facilitating atomicrearrangements. Although Sb is a crosslinking element, it is lesseffective at forming crosslinks than Ge. In light of the reduced Teconcentration, the influence of the increased Sb concentration on thestructural rigidity may not be significant. The instant chalcogenidecompositions may thus represent an optimal balance of the factors thatunderlie the crystallization tendencies of a chalcogenide material.

In one embodiment, the alloy is a material having a Ge concentration inthe range from 11%-22%, an Sb concentration in the range from 22%-65%,and a Te concentration in the range from 28%-55%. In another embodiment,the alloy is a material having a Ge concentration in the range from13%-20%, an Sb concentration in the range from 28%-43%, and a Teconcentration in the range from 43%-55%. In one embodiment, the alloy isa material having a Ge concentration in the range from 15%-18%, an Sbconcentration in the range from 32%-35%, and a Te concentration in therange from 48%-51%.

Illustrative examples of chalcogenide compositions within the scope ofthe instant invention and the characteristics of devices that includethe instant chalcogenide composition are described in the followingexamples.

EXAMPLE 1

In this example, the fabrication of memory devices having activechalcogenide layers in accordance with the instant invention isdescribed. The device structure is a commonly utilized two-terminaldevice design having an active chalcogenide layer in a pore geometry inelectrical contact with top and bottom electrodes. Two different deviceconfigurations were used and similar results were achieved for each.Both designs were deposited on a Si wafer with a thick SiO₂ surfaceoxide layer.

In one design, a chalcogenide layer having a thickness of 500 Å wasdeposited on a circular lower electrode of dimension <1000 Å and havingsurrounding SiO₂ layers. A top electrode was next deposited in situ andincluded a 400 Å carbon layer deposited on top of the chalcogenide layerand one or more conductive layers deposited on top of the carbon layer.The conductive layers typically included a 300 Å TiN layer and a 500 ÅTi layer.

In a second design, a 350 Å bottom electrode layer (e.g. titaniumaluminum nitride) was deposited on the surface oxide layer and aninsulating layer (e.g. SiO₂) was deposited on the bottom electrode. Apore having a diameter of approximately 800 Å was formed in theinsulating layer. A chalcogenide layer having a thickness of 500 Å wasthen deposited. The chalcogenide layer coated the pore and extendedlaterally over the surrounding insulating layer. A top electrode wasnext in situ deposited and included a 400 Å carbon layer deposited ontop of the chalcogenide layer and one or more conductive layersdeposited on top of the carbon layer. The conductive layers typicallyincluded a 300 Å TiN layer and a 500 Å Ti layer. Appropriate lithographyand patterning was performed on each device design to permit addressingof the devices and the devices were subjected to annealing at 300° C.for 30 minutes. Both device designs are well-known in the art andfurther information about chalcogenide phase change memory cells can befound in, for example, U.S. Pat. Nos. 5,166,758; 5,296,716; 5,414,271;5,359,205; and 5,534,712; the disclosures of which are herebyincorporated by reference.

The chalcogenide layer of each memory device of this EXAMPLE wasdeposited at 200° C. using an RF co-sputtering process. Targets ofGe₂Sb₂Te₅, Ge, and Sb were used in the deposition. By controlling thepower, ion energetics, time of exposure and utilization of the differenttargets in the sputtering process, chalcogenide films of differentcomposition were prepared. Memory devices having chalcogenide layerswith the following compositions were fabricated:

Designation Chalcogenide Material Ge (at. %) Sb (at. %) Te (at. %) o5133Ge_(22.2)Sb_(22.2)Te_(55.6) 22.2 22.2 55.6 o5134Ge_(20.0)Sb_(25.5)Te_(54.5) 20.0 25.5 54.5 o5135Ge_(17.8)Sb_(33.3)Te_(48.9) 17.8 33.3 48.9 o5136Ge_(15.6)Sb_(41.1)Te_(43.4) 15.6 41.1 43.4 o5137Ge_(13.3)Sb_(48.8)Te_(37.8) 13.3 48.8 37.8 o5138Ge_(11.1)Sb_(56.6)Te_(32.3) 11.1 56.6 32.3 o5142Ge_(25.0)Sb_(45.5)Te_(29.5) 25.0 45.5 29.5 o5143Ge_(25.2)Sb_(40.7)Te_(35.1) 25.2 40.7 35.1 o5139Ge_(8.9)Sb_(64.4)Te_(26.7) 8.9 64.4 26.7 o5140Ge_(6.7)Sb_(72.2)Te_(21.2) 6.7 72.2 21.2 o5144Ge_(25.0)Sb_(35.5)Te_(39.5) 25.0 35.5 39.5 o5145Ge_(20.0)Sb_(60.5)Te_(19.5) 20.0 60.5 19.5 o5146Ge_(31.0)Sb_(49.5)Te_(19.5) 31.0 49.5 19.5 o5147Ge_(42.0)Sb_(38.5)Te_(19.5) 42.0 38.5 19.5The compositions are listed in atomic percentages of the elementsincluded in the chalcogenide material. The atomic percentages may alsobe referred to herein as the atomic concentration. Many devices usingeach of the chalcogenide compositions were fabricated for this example.The chalcogenide materials and devices containing same may be referredto herein by the composition listed above or by the designation shown inthe left hand column.

The devices of this example are electrical devices that include achalcogenide material, a first terminal in electrical communication withthe chalcogenide material and a second terminal in electricalcommunication with the chalcogenide material, where one or more devicesutilizing each of the chalcogenide compositions indicated above werefabricated. The operational characteristics of the devices arequalitatively similar to the behavior depicted in FIG. 1 as each devicecan be operated among a plurality of reset states (right-side states) ora plurality of accumulation states (left-side states) or a combinationof reset and accumulation states. The different chalcogenidecompositions lead to differences in the operational characteristics ofthe devices and such differences are described in EXAMPLE 2 hereinbelow.

EXAMPLE 2

In this example; the improved crystallization speeds of devices thatinclude chalcogenide materials according to the instant invention aredescribed. The device structures utilized in this example correspond tothose described in EXAMPLE 1 hereinabove. The crystallization speeds ofdevices including several of the chalcogenide compositions listed inEXAMPLE 1 hereinabove were measured. In the measurements, a currentpulse was applied to the device to transform the chalcogenide to aninitial state in the grayscale portion of the response curve. Theresistance of the initial state was recorded. In the next step of theexperiment, energy was applied to the device and the time required toset the device was recorded. The applied energy was in the form of acurrent pulse having a constant amplitude and variable width. The energyof the pulse was such that the device operated in the accumulatingresponse regime of the resistance vs. current plot of the device (seeFIG. 1). The resistance of the device was monitored as a function of thetime of application of the pulse to the device with variable pulse widthfrom 20 ns to 5 μs investigated. The setting transformation wassignified by the decrease in resistance described in connection withFIG. 1 hereinabove. The pulse time required to achieve the set state wasrecorded. For each device, the experiment was repeated by establishingseveral different initial states in the grayscale regime extending overa wide variety of resistances and the relationship between theresistance of the initial state and the pulse time required to set thedevice was determined.

FIG. 2 shows the dependence of the pulse time required to set the deviceon the resistance of the initial state used in the experiment. The pulsetime required to set the device may also be referred to as the set pulsewidth and is designated as Wset in FIG. 2. The set pulse time isreported in units of seconds. The resistance of the initial state usedin the experiment may also be referred to as a reset resistance of thedevice since it represents the resistance of the grayscale state towhich the device was reset prior to initiation of the experiment.(Within the framework of this terminology, the reset state having themaximum resistance may be referred to as the saturated reset state.) Theresistance of the initial state is designated as Rrs in FIG. 2 and isreported in units of Ohms. FIG. 2 shows several data curves. Each datacurve corresponds to a device that includes a different chalcogenidecomposition and the points on each curve correspond to different resetstates for the device. The legend of FIG. 2 identifies the chalcogenidecomposition associated with each data curve by its designation in thetable of compositions presented hereinabove. The data curve designatedas “Control” refers to a device that includes the prior art Ge₂Sb₂Te₅composition.

The data curve for the control device is typical of the response ofprior art chalcogenide materials. When the resistance of the reset stateof the control device is below about 4.5 kΩ, the set pulse time is about20 ns. Above about 4.5 kΩ, however, the set pulse time increasesdramatically, reaching a value of about 400 ns at a resistance of about11.9 kΩ. The results show an approximate twenty-fold difference in setpulse time for memory states that differ in resistance by a factor ofslightly greater than two. In practical memory applications, aresistance ratio between memory states of at least a factor of two isdesired to permit reliable discrimination of the different states duringread out. In the case of the control device, the data show that asignificant increase in set pulse time accompanies a two-fold increasein resistance. In terms of operational speed, the longer set pulse timeof the higher resistance state is the controlling factor.

A consideration of the data curves for devices that include alloysaccording to the instant invention show a much more favorablerelationship between set pulse time and reset resistance. The datacurves of devices including the instant alloys generally fallsignificantly below the data curve of the control device. Devices thatinclude the instant alloys offer the beneficial characteristic ofproviding short set pulse times for states having higher resetresistances. Representative data points from selected data curves ofFIG. 2 are summarized in the table below:

Data Curve Composition Rrs (kΩ) Wset (ns) o5134Ge_(20.0)Sb_(25.5)Te_(54.5) 6.3 24 o5134 Ge_(20.0)Sb_(25.5)Te_(54.5) 8.360 o5134 Ge_(20.0)Sb_(25.5)Te_(54.5) 245 149 o5135Ge_(17.8)Sb_(33.3)Te_(48.9) 11 20 o5135 Ge_(17.8)Sb_(33.3)Te_(48.9) 10324 o5135 Ge_(17.8)Sb_(33.3)Te_(48.9) 252 86 o5136Ge_(15.6)Sb_(41.1)Te_(43.4) 9.7 29 o5136 Ge_(15.6)Sb_(41.1)Te_(43.4) 3733 o5136 Ge_(15.6)Sb_(41.1)Te_(43.4) 175 60 o5136Ge_(15.6)Sb_(41.1)Te_(43.4) 296 124 o5137 Ge_(13.3)Sb_(48.8)Te_(37.8) 2120 o5137 Ge_(13.3)Sb_(48.8)Te_(37.8) 94 50 o5138Ge_(11.1)Sb_(56.6)Te_(32.3) 6.6 20 o5138 Ge_(11.1)Sb_(56.6)Te_(32.3) 3420 o5138 Ge_(11.1)Sb_(56.6)Te_(32.3) 48 60

The data points illustrate a clear advantage for the instant alloys overthe prior art control alloy. Short set pulse times are observed at muchhigher reset resistances (and over a much wider range or resetresistances) in the devices containing the instant alloys. In the devicethat included the alloy Ge_(17.8)Sb_(33.3)Te_(48.9), for example, agreater than twenty-fold increase in resistance was accompanied by anincrease in set pulse time of a factor of only 4.3. Comparably favorableresults are observed for other alloys disclosed herein, including thoselisted in the table above.

Devices that operate with the instant alloys thus exhibit faster settransformations over a wider range of reset resistances than analogousdevices that operate with prior art alloys. From an applicationstandpoint, the extended range of resistances over which fast settransformations are observed is beneficial because it permits theoperation of binary devices using memory states widely separated inresistance without sacrificing operational speed. In the case of thecontrol device, for example, a set pulse time of 400 nspermits-operation between memory states separated in resistance by afactor of only slightly greater than two. In the case of a device thatincludes Ge_(17.8)Sb_(33.3)Te_(48.9), in contrast, a set pulse time ofonly 86 ns permits operation between memory states separated inresistance by a factor of more than 20. Operating between memory stateswidely separated in resistance is desirable because such states are moreeasily distinguished upon reading and more tolerant in cell-to-cellprogramming variation. A greater resistance contrast reduces readerrors.

Devices that include the instant alloys are also advantageous inmultistate memory applications. The extended range of resistance statesover which fast set transformations occur for the instant alloys meansthat operation with more memory states becomes possible with lesssacrifice of device speed. If, for example, a minimum resistancecontrast of about a factor of two is desired to insure sufficientlyaccurate readability of memory states, only two memory states areavailable in the control device if one wishes to operate at a ratelimited by a set pulse of 400 ns. In the case of the device thatincludes Ge_(17.8)Sb_(33.3)Te_(48.9) alloy, on the other hand, fivememory states, consecutive ones of which exhibit a resistance contrastof a factor of two, can be defined for operation at the faster ratedefined by a set pulse time of 86 ns.

The instant invention provides an electrical device that includes achalcogenide material in electrical communication with at least twoterminals, where the device can be operated between a plurality ofstates determined by the structural characteristics of the chalcogenidematerial. In one embodiment, the operational states of the deviceinclude two or more reset states where the resistance of one of thereset states is greater than the resistance of the other of the resetstates by a factor of at least 3 and the required set pulse time of thehigher resistance state is greater than the required set pulse time ofthe lower resistance reset state by a factor of less than 20. In anotherembodiment, the operational states of the device include two or morereset states where the resistance of one of the reset states is greaterthan the resistance of the other of the reset states by a factor of atleast 3 and the required set pulse time of the higher resistance stateis greater than the required set pulse time of the lower resistancereset state by a factor of less than 10. In another embodiment, theoperational states of the device include two or more reset states wherethe resistance of one of the reset states is greater than the resistanceof the other of the reset states by a factor of at least 3 and therequired set pulse time of the higher resistance state is greater thanthe required set pulse time of the lower resistance reset state by afactor of less than 5. In another embodiment, the operational states ofthe device include two or more reset states where the resistance of oneof the reset states is greater than the resistance of the other of thereset states by a factor of at least 3 and the required set pulse timeof the higher resistance state is greater than the required set pulsetime of the lower resistance reset state by a factor of less than 2.

In one embodiment, the operational states of the device include two ormore reset states where the resistance of one of the reset states isgreater than the resistance of the other of the reset states by a factorof at least 10 and the required set pulse time of the higher resistancestate is greater than the required set pulse time of the lowerresistance reset state by a factor of less than 20. In anotherembodiment, the operational states of the device include two or morereset states where the resistance of one of the reset states is greaterthan the resistance of the other of the reset states by a factor of atleast 10 and the required set pulse time of the higher resistance stateis greater than the required set pulse time of the lower resistancereset state by a factor of less than 10. In another embodiment, theoperational states of the device include two or more reset states wherethe resistance of one of the reset states is greater than the resistanceof the other of the reset states by a factor of at least 10 and therequired set pulse time of the higher resistance state is greater thanthe required set pulse time of the lower resistance reset state by afactor of less than 5.

In one embodiment, the operational states of the device include two ormore reset states where the resistance of one of the reset states isgreater than the resistance of the other of the reset states by a factorof at least 20 and the required set pulse time of the higher resistancestate is greater than the required set pulse time of the lowerresistance reset state by a factor of less than 20. In anotherembodiment, the operational states of the device include two or morereset states where the resistance of one of the reset states is greaterthan the resistance of the other of the reset states by a factor of atleast 20 and the required set pulse time of the higher resistance stateis greater than the required set pulse time of the lower resistancereset state by a factor of less than 10. In another embodiment, theoperational states of the device include two or more reset states wherethe resistance of one of the reset states is greater than the resistanceof the other of the reset states by a factor of at least 20 and therequired set pulse time of the higher resistance state is greater thanthe required set pulse time of the lower resistance reset state by afactor of less than 5.

FIG. 3 was derived from FIG. 2 and summarizes selected set pulse widthdata for the control device and devices that include the instant alloys.FIG. 3 specifically shows the set pulse time from a reset state having aresistance above 100 kΩ for devices including the control alloy andseveral of the instant alloys. The set state in the transformation ofeach of the devices had a resistance of less than 5 kΩ. In FIG. 3, theset pulse width is plotted as a function of the atomic percentages ofGe, Sb and Te in the active chalcogenide layer of the devices. Theatomic percentages of Ge, Sb, and Te are depicted with diamond symbols,triangle symbols, and square symbols, respectively. The atomicpercentage of each element is indicated for each of severalcompositions, so that each composition is represented by three symbolsin FIG. 3. Since one set pulse width is reported for each composition inFIG. 3, the three symbols representing each composition arehorizontally-aligned. The uppermost set of three symbols correspond tothe set pulse width of the control device, which utilizes Ge₂₂Sb₂₂Te₅₅as the active chalcogenide material. This composition exhibits thelongest set pulse width displayed in FIG. 3.

Also included in FIG. 3 are three ovals that designate the preferredatomic percentage of each element of the chalcogenide material. Theapproximate ranges associated with the ovals are based on thedesirability of realizing short set pulse widths and fast deviceoperation. The left oval depicts the preferred range of the atomicpercentage of Ge and extends from about 13.5% to about 18%. In thisrange of Ge compositions, the set pulse width of the device isconsiderably shorter than the set pulse width of the control device(which includes a chalcogenide material having an atomic percentage ofGe of 22%) and of a device that includes Ge₂₀Sb₃₀Te₅₀ as the activechalcogenide material.

The middle oval depicts the preferred range of the atomic percentage ofSb and extends from about 33.0% to about 41%. In this range of Sbcompositions, the set pulse width of the device is considerably shorterthan the set pulse width of the control device (which includes achalcogenide material having an atomic percentage of Sb of 22%) and of adevice that includes Ge_(20.0)Sb_(25.5)Te_(54.5) as the activechalcogenide material.

The right oval depicts the preferred range of the atomic percentage ofTe and extends from about 37% to about 48%. In this range of Tecompositions, the set pulse width of the device is considerably shorterthan the set pulse width of the control device (which includes achalcogenide material having an atomic percentage of Te of 55%) and of adevice that includes Ge_(20.0)Sb_(25.5)Te_(54.5) as the activechalcogenide material.

FIG. 4 is also derived from the results presented in FIG. 2 and providesa summary of the required reset current of the devices. The resetcurrent is presented as a function of the atomic percentages of theelements included in the active chalcogenide layer of the devices. As inFIG. 3, the atomic percentage of Ge, Sb, and Te is depicted for eachcomposition and each composition is represented by threehorizontally-aligned symbols. The atomic percentages of Ge, Sb, and Teare depicted with diamonds, triangles, and squares, respectively. Thereset current is expressed as amperes (A) and corresponds to the currentrequired to transform the device to its saturated reset state. Asdescribed hereinabove, the saturated reset state of a device is thereset state having maximum resistance. In order to minimize the powerrequired to operate the device, it is desirable for the device to have alow reset current.

FIG. 4 includes ovals to indicate the preferred range of the atomicpercentages of Ge, Sb, and Te. The preferred ranges correspond to atomicpercentages of the different elements that lead to lower reset currents.The left oval depicts the preferred range of the atomic percentage of Geand extends from about 14% to about 22%. In this range of Gecompositions, the reset current of the device is generally reducedrelative to compositions having a greater or lesser atomic percentage ofGe. The middle oval depicts the preferred range of the atomic percentageof Sb and extends from about 17% to about 33%. In this range of Sbcompositions, the reset current of the device is generally reducedrelative to compositions having a greater or lesser atomic percentage ofSb. The right oval depicts the preferred range of the atomic percentageof Te and extends from about 43% to about 55%. In this range of Tecompositions, the reset current of the device is generally reducedrelative to compositions having a greater or lesser atomic percentage ofTe.

In addition to the results described in EXAMPLE 2 hereinabove, furtherexperiments using the devices and compositions described in EXAMPLE 1hereinabove were completed. These experiments were directed at themeasurements of: the set pulse width necessary to transform a devicefrom a reset state having a resistance of 50 kΩ to a set state having aresistance below 5 kΩ; the threshold voltage of the device in itssaturated reset state; the holding voltage of the device; and the virginresistance of the device. The set of parameters included in theseexperiments and the experiments described in EXAMPLE 2 hereinabovecorrespond to several of the more important device characteristics forpractical memory applications. The result generally indicate a smallvariation in the optimal range of atomic percentages of Ge, Sb, and Tefor the different properties. As a result, in the design of new devices,the significance of the different properties are weighed against eachother to achieve an overall optimum level of performance.

The instant invention generally provides chalcogenide materials thatinclude Ge and Sb. In one embodiment, the atomic concentration of Ge isbetween 11% and 21%. In a preferred embodiment, the atomic concentrationof Ge is between 13% and 20%. In another preferred embodiment, theatomic concentration of Ge is between 15% and 18%. In one embodiment,the atomic concentration of Sb is between 22% and 65%. In a preferredembodiment, the atomic concentration of Sb is between 28% and 43%. Inanother preferred embodiment, the atomic concentration of Sb is between32% and 35%. In each of the foregoing embodiments, the compositionranges indicated for each of the elements is inclusive of the endpointcompositions.

The instant invention further provides chalcogenide materials thatinclude Ge and Sb in the concentration ranges described above as well asTe. In one embodiment, the atomic concentration of Te is between 28% and55%. In a preferred embodiment, the atomic concentration of Te isbetween 43% and 55%. In another preferred embodiment, the atomicconcentration of Te is between 48% and 51%. In each of the foregoingembodiments, the composition ranges indicated for each of the elementsis inclusive of the endpoint compositions.

The instant invention further includes embodiments having functionalequivalents to the illustrative embodiments described hereinabove. Asdescribed in several of the U.S. Patents incorporated by referencehereinabove, chalcogenide materials generally include a chalcogenelement and one or more chemical or structural modifying elements. Thechalcogen element (e.g. Te, Se) is selected from column VI of theperiodic table and the modifying elements can be selected from columnIII (e.g. Ga, Al, In), column IV (e.g. Si, Ge, Sn), or column V (e.g. P,As, Sb) of the periodic table. The role of modifying elements includesproviding points of branching or crosslinking between chains comprisingthe chalcogen element. Column IV modifiers can function astetracoordinate modifiers that include two coordinate positions within achalcogenide chain and two coordination positions that permit branchingor crosslinking away from the chalcogenide chain. Column III and Vmodifiers can function as tricoordinate modifiers that include twocoordinate positions within a chalcogenide chain and one coordinateposition that permits branching or crosslinking away from thechalcogenide chain. Although the embodiments described hereinabove haveillustrated the features of the instant invention using chalcogenidematerials that include Ge, Sb, and/or Te, it is to be understood bythose of skill in the art that Ge may be substituted in whole or in partwith another column IV element (e.g. Si), Sb may be substituted in wholeor in part with another column V element (e.g. As), and Te may besubstituted in whole or in part with another column VI element (e.g.Se).

In addition to individual devices, the instant invention further extendsto arrays of devices. The instant chalcogenide materials and devices canbe integrated into arrays, including X-Y arrays, such as those describedin U.S. Pat. Nos. 5,694,146; 5,912,839; and 6,141,241; the disclosuresof which are hereby incorporated by reference. Chalcogenide devicearrays may be used for both memory and processing capabilities;including logic and parallel computing.

The foregoing discussion and description are not meant to be limitationsupon the practice of the present invention, but rather illustrativethereof. It is to be appreciated by persons of skill in the art thatnumerous equivalents of the illustrative embodiments disclosed hereinexist. It is the following claims, including all equivalents and obviousvariations thereof, in combination with the foregoing disclosure whichdefine the scope of the invention.

1. A chalcogenide material comprising Ge, Sb, and Te, said chalcogenidematerial including a crystalline phase portion and an amorphous phaseportion, said chalcogenide material having a first state with a firstvolume fraction of said crystalline phase portion and a second statewith a second volume fraction of said crystalline phase portion, saidfirst state being transformable to said second state upon application ofelectrical energy, wherein the atomic concentration of Ge is between 15%and 18%, the atomic concentration of Sb is between 25% and 49% and theatomic concentration of Te is between 43% and 49%.
 2. The chalcogenidematerial of claim 1, wherein the atomic concentration of Sb is between28% and 43%.
 3. The chalcogenide material of claim 1, wherein the atomicconcentration of Sb is between 32% and 35%.
 4. The chalcogenide materialof claim 1, wherein the atomic concentration of Te is between 43% and55%.
 5. The chalcogenide material of claim 3, wherein the atomicconcentration of Te is between 48% and 51%.
 6. An electrical devicecomprising the chalcogenide material of claim 1, a first electrode inelectrical communication with said chalcogenide material, and a secondelectrode in electrical communication with said chalcogenide material.7. The chalcogenide material of claim 2, wherein the atomicconcentration of Sb is between 33% and 41%.
 8. The chalcogenide materialof claim 1, wherein the resistance of said first state is greater thanthe resistance of said second state by a factor of at least 3 and therequired set pulse time of said first state is greater than the requiredset pulse time of said second state by a factor of less than
 20. 9. Thechalcogenide material of claim 8, wherein the required set pulse time ofsaid first state is greater than the required set pulse time of saidsecond state by a factor of less than
 5. 10. The chalcogenide materialof claim 8, wherein the resistance of said first state is greater thanthe resistance of said second state by a factor of at least
 10. 11. Thechalcogenide material of claim 10, wherein the required set pulse timeof said first state is greater than the required set pulse time of saidsecond state by a factor of less than
 10. 12. The chalcogenide materialof claim 10, wherein the required set pulse time of said first state isgreater than the required set pulse time of said second state by afactor of less than
 5. 13. The chalcogenide material of claim 8, whereinthe resistance of said first state is greater than the resistance of thesecond state by a factor of at least
 20. 14. The chalcogenide materialof claim 13, wherein the required set pulse time of said first state isgreater than the required set pulse time of said second state by afactor of less than
 10. 15. The chalcogenide material of claim 13,wherein the required set pulse time of said first state is greater thanthe required set pulse time of said second state by a factor of lessthan 5.